【十八掌●基本功篇】第一掌：Java之多线程--锁

这一篇博文是【大数据技术●降龙十八掌】系列文章的其中一篇，点击查看目录：大数据技术●降龙十八掌

synchronized可以实现同步访问，java.util.concurrent.locks包下的Lock也可以实现同步访问。
synchronized是java中的一个关键字，是Java语言内置的特性。处于synchronized作用下的代码块或者方法代码块，是不允许两个线程同时进入，不管是读操作还是写操作，都是不允许同时进行的，这在高并发的情况下对性能影响很大。而Lock更复杂，控制也更精细，另外Lock提供的功能更丰富。

1、Lock

Lock是一个接口，定义如下：

public interface Lock {void lock();void lockInterruptibly() throws InterruptedException;boolean tryLock();boolean tryLock(long time, TimeUnit unit) throws InterruptedException;void unlock();Condition newCondition();
}

2、ReadWriteLock

ReadWriteLock是一个接口，它定义了读锁和写锁，可以使得多个线程可以同时读。定义如下：

public interface ReadWriteLock {Lock readLock();Lock writeLock();
}

3、ReentrantReadWriteLock

ReentrantReadWriteLock是个类，它实现了ReadWriteLock接口。在应用读锁和写锁时候，如果一个线程占用了写锁，那么其他线程不能获取写锁也不能占用读锁，只有写完其他线程才能读和写，当一个线程占用了读锁，那么其他线程可以获取读锁，但是不能占用写锁，也就是只能读不能写。这样就比synchronized提高了读取时候的效率，因为synchronized作用下，不管读取还是写入，都必须一个线程一个线程地进行。

下面是一个ReentrantReadWriteLock的应用实例，模拟多个客户端对同一个银行账号进行操作，操作有：存钱、取钱、查询余额，保证存钱和取钱这两个写操作不能同时进行，但是在没有写入的时候，多个查询余额操作可以同时进行。

package lock;import java.util.Date;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;/*** Created by 鸣宇淳 on 2017/12/19.* 多客户端对银行账号进行存取查询的例子*/
public class MyLockDemo {public static void main(String[] args) throws InterruptedException {//创建银行账户Account account = new Account("6222022222212154578", 1000);//创建一个锁ReadWriteLock lock = new ReentrantReadWriteLock(false);//三个客户端同时操作Client client1 = new Client("存钱1", 1, account, 500, lock);Client client2 = new Client("取钱2", 2, account, 800, lock);Client client3 = new Client("查询3", 3, account, 0, lock);Client client4 = new Client("查询4", 3, account, 0, lock);Client client5 = new Client("存钱5", 1, account, 100, lock);Thread t1 = new Thread(client1);Thread t2 = new Thread(client2);Thread t3 = new Thread(client3);Thread t4 = new Thread(client4);Thread t5 = new Thread(client5);System.out.println(new Date() + "账户内起始金额为1000\n---------------");t1.start();t2.start();t3.start();t4.start();t5.start();}
}package lock;import java.util.Date;
import java.util.concurrent.locks.ReadWriteLock;/*** Created by 鸣宇淳 on 2017/12/19.、* 存取客户端*/
public class Client implements Runnable {private String name; //客户端操作人private int saveOrWithdrawOrGet;//1.存钱  2.取钱  3.查询余额private ReadWriteLock lock;//锁private Account account;//操作账号private int num;//存取金额public Client(String name, int saveOrWithdrawOrGet, Account account, int num, ReadWriteLock lock) {this.name = name;this.saveOrWithdrawOrGet = saveOrWithdrawOrGet;this.lock = lock;this.account = account;this.num = num;}public void run() {if (this.saveOrWithdrawOrGet == 1) {//存钱try {System.out.println(new Date() +this.name + "to....申请锁存钱");lock.writeLock().lock();int r = account.saveMoney(this.num);System.out.println(new Date() +this.name + "存钱" + this.num + ",余额为" + r);Thread.sleep(2000);} catch (InterruptedException e) {e.printStackTrace();} finally {lock.writeLock().unlock();}} else if (this.saveOrWithdrawOrGet == 2) {//取钱try {System.out.println(new Date() +this.name + "to....申请锁取钱");lock.writeLock().lock();int r = account.withdrawMoney(num);System.out.println(new Date() + this.name + "取钱" + this.num + ",余额为" + r);Thread.sleep(2000);} catch (InterruptedException e) {e.printStackTrace();} finally {lock.writeLock().unlock();}} else if (this.saveOrWithdrawOrGet == 3) {//查询余额try {System.out.println(new Date() +this.name + "to....申请锁查询");lock.readLock().lock();int r = account.getMoney();System.out.println(new Date() +this.name + "查询余额，余额为" + r);Thread.sleep(2000);} catch (InterruptedException e) {e.printStackTrace();} finally {lock.readLock().unlock();}}}
}package lock;/*** Created by 鸣宇淳 on 2017/12/19.* 银行账户*/
public class Account {private String cardcode;private int money;public Account(String cardcode, int money) {this.cardcode = cardcode;this.money = money;}//存钱public int saveMoney(int num) {this.money = this.money + num;return this.money;}//取钱public int withdrawMoney(int num) {if (num <= this.money) {this.money = this.money - num;}return this.money;}//查询余额public int getMoney() {return this.money;}
}

运行以上代码，输入如下：

Thu Dec 28 11:09:22 CST 2017账户内起始金额为1000
---------------
Thu Dec 28 11:09:22 CST 2017取钱2to....申请锁取钱
Thu Dec 28 11:09:22 CST 2017存钱5to....申请锁存钱
Thu Dec 28 11:09:22 CST 2017查询3to....申请锁查询
Thu Dec 28 11:09:22 CST 2017查询4to....申请锁查询
Thu Dec 28 11:09:22 CST 2017存钱1to....申请锁存钱
Thu Dec 28 11:09:22 CST 2017存钱5存钱100,余额为1100
Thu Dec 28 11:09:24 CST 2017取钱2取钱800,余额为300
Thu Dec 28 11:09:26 CST 2017查询3查询余额，余额为300
Thu Dec 28 11:09:26 CST 2017查询4查询余额，余额为300
Thu Dec 28 11:09:28 CST 2017存钱1存钱500,余额为800

通过输出可以看到，客户端12345同时申请各自的锁，存储5客户端11:09:22首先抢到了写锁，进行存钱操作后休眠了2秒，这个过程中，其他线程不管是申请写锁或者读锁，都不能得到锁，2秒后11:09:24取钱2客户端抢到了写锁，进程取钱操作后，休眠2秒，这个过程中同样其他线程不能读也不能写，11:09:26查询3客户端得到读锁，然后也休眠2秒，这个过程，查询4可以得到赌说，但是存钱1不能得到写锁，也就是只能读不能写，只有当2秒后，查询3和查询4都释放了读锁后，存钱1才能抢到写锁，进行存钱操作。